feat(tvix/eval): add warning for deprecated URL literals

Change-Id: I8a9cfcb5d99680344ee0e3461a4a52526b846c92
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6175
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/warnings.rs

@@ -2,7 +2,9 @@
 /// problems that the user could address.
 
 #[derive(Debug)]
-pub enum WarningKind {}
+pub enum WarningKind {
+    DeprecatedLiteralURL,
+}
 
 #[derive(Debug)]
 pub struct EvalWarning {